U.S. patent number 5,450,812 [Application Number 08/163,427] was granted by the patent office on 1995-09-19 for process for growing a film epitaxially upon an oxide surface and structures formed with the process.
This patent grant is currently assigned to Martin Marietta Energy Systems, Inc.. Invention is credited to Rodney A. McKee, Frederick J. Walker.
United States Patent |
5,450,812 |
McKee , et al. |
September 19, 1995 |
Process for growing a film epitaxially upon an oxide surface and
structures formed with the process
Abstract
A process and structure wherein a film comprised of a perovskite
or a spinel is built epitaxially upon a surface, such as an
alkaline earth oxide surface, involves the epitaxial build up of
alternating constituent metal oxide planes of the perovskite or
spinel. The first layer of metal oxide built upon the surface
includes a metal element which provides a small cation in the
crystalline structure of the perovskite or spinel, and the second
layer of metal oxide built upon the surface includes a metal
element which provides a large cation in the crystalline structure
of the perovskite or spinel. The layering sequence involved in the
film build up reduces problems which would otherwise result from
the interfacial electrostatics at the first atomic layers, and
these oxides can be stabilized as commensurate thin films at a unit
cell thickness or grown with high crystal quality to thicknesses of
0.5-0.7 .mu.m for optical device applications.
Inventors: |
McKee; Rodney A. (Kingston,
TN), Walker; Frederick J. (Oak Ridge, TN) |
Assignee: |
Martin Marietta Energy Systems,
Inc. (Oak Ridge, TN)
|
Family
ID: |
22589967 |
Appl.
No.: |
08/163,427 |
Filed: |
December 8, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
100743 |
Jul 30, 1993 |
|
|
|
|
Current U.S.
Class: |
117/84; 117/104;
427/248.1; 438/967; 117/105 |
Current CPC
Class: |
C30B
23/02 (20130101); G02B 6/131 (20130101); C30B
29/32 (20130101); C30B 29/16 (20130101); C30B
29/22 (20130101); Y10S 438/967 (20130101) |
Current International
Class: |
C30B
23/02 (20060101); G02B 6/13 (20060101); C30B
023/02 () |
Field of
Search: |
;117/84,104 ;427/248.1
;428/468 ;437/105 ;501/2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Breneman; R. Bruce
Assistant Examiner: Garrett; Felisa
Attorney, Agent or Firm: McKee; Michael E. Craig; George L.
Spicer; James M.
Parent Case Text
This application is a continuation-in-part of application Ser. No.
08/100,743 filed Jul. 30, 1993 and entitled PROCESS FOR GROWING A
FILM EPITAXIALLY UPON AN MgO SURFACE AND STRUCTURES FORMED WITH THE
PROCESS, the disclosure of which is incorporated herein by
reference.
Claims
We claim:
1. A process for coating a body with an epitaxial film wherein the
body has a surface provided by an alkaline earth oxide, the process
comprising the steps of:
growing, by molecular beam epitaxy (MBE) techniques, a single plane
of metal oxide having oxygen and a metal element of a group of
metals consisting of Ti, Zr, Hf, V, Cr, Mn, Fe, Co, Ni and Cu upon
the alkaline earth oxide surface wherein the oxygen and metal atoms
of the metal oxide are deposited upon the alkaline earth oxide
surface in quantities sufficient to construct a single plane of the
metal oxide and come to rest during the deposition process at
ordered sites across the alkaline earth oxide surface.
2. The process as defined in claim 1 wherein the metal element of
the single metal oxide plane provides a relatively small cation
with respect to the size of the oxygen in the crystalline form of
the metal oxide and the step of growing the single plane of the
metal oxide is followed by a step of
growing, by molecular beam epitaxy (MBE) techniques, a constituent
metal oxide plane of a perovskite crystal or a spinel crystal
epitaxially upon the single plane of metal oxide wherein the
perovskite or spinel has a crystalline form comprised of two metal
oxide planes, and wherein the metal oxide of one of the two metal
oxide planes of the perovskite or spinel crystalline form includes
one metal which provides a small cation in the perovskite or spinel
crystalline structure and the metal oxide of the other of the two
metal oxide planes of the perovskite or spinel crystalline form
includes another metal which provides a large cation in the
perovskite or spinel crystalline structure, and wherein the
constituent metal oxide plane grown by this step includes the metal
which provides the large cation in the perovskite or spinel
crystalline structure.
3. The process as defined in claim 2 wherein the constituent metal
oxide plane provides a first epitaxial layer and the step of
growing the first layer is followed by the step of:
growing, by MBE techniques, a second epitaxial layer upon the first
epitaxial layer wherein the second epitaxial layer is comprised of
a constituent metal oxide plane of a perovskite crystal or a spinel
crystal wherein the constituent metal oxide plane of the second
epitaxial layer includes the metal which provides the small cation
in the perovskite or spinel crystalline structure.
4. A process as defined in claim 3 wherein the metal oxide of the
second epitaxial layer is a metal oxide having a metal element of a
group of metals consisting of Ti, Zr, Hf, V, Cr, Mn, Fe, Co, Ni and
Cu.
5. A process as defined in claim 3 wherein the step of growing the
second epitaxial layer is followed by the repeating in sequence
of:
growing another constituent metal oxide plane of a perovskite
crystal or a spinel crystal upon the second epitaxial layer wherein
the metal of said another metal oxide plane provides the large
cation in the perovskite or spinel crystalline structure, and
then growing a further constituent metal oxide plane of a
perovskite crystal or a spinel crystal upon said another
constituent metal oxide plane wherein the metal of another metal
oxide plane provides the small cation in the perovskite or spinel
crystalline structure.
6. The process as defined in claim 3 wherein the step of growing
the second epitaxial layer is followed by the sequential steps
of:
a) growing a single epitaxial plane of metal oxide having a metal
element of a group of metals consisting of Ti, Zr, Hf, V, Cr, Mn,
Fe, Co, Ni and Cu upon the second epitaxial layer; and
b) growing a single epitaxial plane of metal oxide directly upon
the single epitaxial plane of metal oxide grown in step a) wherein
the metal oxide plane grown in this step b) is a constituent metal
oxide plane of a perovskite crystal or a spinel crystal and
includes the metal which provides the small cation in the
perovskite or spinel crystalline structure.
7. The process as defined in claim 6 wherein steps a) and b) are
repeated, inclusively, until no lattice strain within the layup of
planes appears at the surface of the layup.
8. The process as defined in claim 6 wherein steps a) and b) are
repeated, inclusively, until the total number of constituent metal
oxide planes grown by step a) is at least twenty-five.
9. The process as defined in claim 8 wherein the twenty-fifth plane
of constituent metal oxide grown by step a) is followed by the
steps of:
growing, by MBE techniques, layers of a perovskite or spinel upon
the grown layup of planes wherein the perovskite or spinel layers
are grown in a layer-by-layer fashion.
10. A process for coating a body with an epitaxial film wherein the
body has a surface defined by a metal oxide provided by either a
Group IVA element oxide or an oxide constituent of a perovskite
crystal or a spinel crystal wherein the metal element of the metal
oxide provides a relatively small cation in the crystalline form of
the metal oxide and the metal and oxygen atoms of the metal oxide
are disposed at ordered sites across the oxide surface, the process
comprising the steps of:
growing, by molecular beam epitaxy (MBE) techniques, a constituent
metal oxide plane of a perovskite crystal or a spinel crystal
epitaxially upon the single plane of metal oxide wherein the metal
element of the constituent metal oxide plane provides a relatively
large cation in the perovskite or spinel crystalline structure.
11. The process as defined in claim 10 wherein the constituent
metal oxide plane provides a first epitaxial layer and the step of
growing the first layer is followed by the step of:
growing, by MBE techniques, a second epitaxial layer upon the first
epitaxial layer wherein the second epitaxial layer is comprised of
a constituent metal oxide plane of a perovskite crystal or a spinel
crystal and includes the metal which provides a relatively small
cation in the perovskite or spinel crystalline structure.
12. A process as defined in claim 11 wherein the step of growing
the second epitaxial layer is followed by the repeating in sequence
the steps of:
growing another constituent metal oxide plane of a perovskite
crystal or a spinel crystal upon the second epitaxial layer wherein
the metal element of said another metal oxide plane provides the
large cation in the perovskite or spinel crystalline structure,
and
then growing a further constituent metal oxide plane of a
perovskite or spinel crystal upon said another constituent metal
oxide plane wherein the metal of said another metal oxide plane
provides the small cation in the perovskite or spinel crystalline
structure.
13. The process as defined in claim 11 wherein the step of growing
the second epitaxial layer is followed by the sequential steps
of:
a) growing a single epitaxial plane of metal oxide having a metal
element of a group of metals consisting of Ti, Zr, Hf, V, Cr, Mn,
Fe, Co, Ni and Cu upon the second epitaxial layer; and
b) growing a single epitaxial plane of metal oxide directly upon
the single epitaxial plane of metal oxide grown in step a) wherein
the metal oxide plane grown in this step b) is a constituent metal
oxide plane of a perovskite crystal or a spinel crystal and
includes the metal which provides the small cation in the
perovskite or spinel crystalline structure.
14. The process as defined in claim 13 wherein steps a) and b) are
repeated, inclusively, until no lattice strain within the layup of
planes appears at the surface of the layup.
15. The process as defined in claim 13 are repeated, inclusively,
until the total number of constituent metal oxide planes grown by
step a) is at least twenty-five.
16. The process as defined in claim 15 wherein the twenty-fifth
plane of constituent metal oxide grown by step a) is followed by
the steps of:
growing, by MBE techniques, layers of a perovskite crystal or a
spinel upon the grown layup of planes wherein the perovskite or
spinel layers are grown in a layer-by-layer fashion.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to the preparation of structures
for use in a semiconductor and/or optical wave guide applications
and relates, more particularly, to the growth of an epitaxial film
upon surfaces, such as an alkaline earth oxide surface.
Oxides in a class of oxides known as perovskites and spinels are
known to exhibit technologically-significant properties, such as
ferroelectricity, ferromagnetism, piezoelectricity,
superconductivity and nonlinear electro-optic behavior, and for
this reason, are grown upon substrates for the purpose of
incorporating these properties within electronic devices. With such
oxides grown upon substrates, the aforementioned properties can be
taken advantage of in a number of devices, and in particular, are
believed to be well-suited for use in Faraday Rotators for optical
isolators and in magnetic memory applications.
Of these electronic devices, optical guided wave (OGW) devices
constructed with perovskites are relatively demanding from the
standpoint of optical clarity and necessarily require long range
structural coherence. Heretofore, the optical clarity and
structural coherence of a perovskite film grown upon an alkaline
earth oxide, such as MgO, has been limited due, at least in part,
to the inability to grow a perovskite upon the alkaline earth
wherein the grown perovskite is of a single orientation. It would
be desirable to provide a process for growing perovskite of
single-orientation upon an alkaline earth oxide and thus enhance
the quality of the resulting structure for OGW applications.
Accordingly, an object of the present invention is to provide a new
and improved process for growing a perovskite or a spinel of single
orientation on an alkaline earth oxide and structures formed with
the process.
Another object of the present invention is to provide such a
process which is well-suited for coating an alkaline earth oxide
surface with a single layer of a Group IVA element oxide, i.e.
TiO.sub.2, ZrO.sub.2 or HfO.sub.2.
Still another object of the present invention is to provide such a
structure which is well-suited for use in an OGW applications or
for incorporation within an integrated circuit.
Yet another object is to provide a new and improved process for
growing a perovskite or a spinel or constituents of a perovskite or
spinel spitaxially upon a surface provided by a Group IVA element
oxide or an oxide constituent of a perovskite or a spinel and
structures formed with the process.
A further object of the present invention is to provide such a
structure whose ferromagnetic properties render it well-suited for
use in magneto-optic applications.
SUMMARY OF THE INVENTION
This invention resides in a process for coating a body with an
spitaxial film wherein the body has a surface provided by one of an
alkaline earth oxide, a Group IVA element oxide, an oxide
constituent of a perovskite and an oxide constituent of a spinel
and structures formed with the process.
One embodiment of the process includes the steps of growing, by
molecular beam epitaxy (MBE) techniques, a single plane of metal
oxide having a metal element of a group of metals consisting of Ti,
Zr, Hf, V, Cr, Mn, Fe, Co, Ni and Cu upon a surface provided by an
alkaline earth oxide so that the metal and oxygen atoms of the
single plane are disposed at ordered sites across the alkaline
earth oxide surface. In a further embodiment of the method, the
step of growing a single plane of metal oxide of the aforementioned
group of oxides is followed by the step of growing, by MBE
techniques, a constituent metal oxide plane of a perovskite or a
spinel upon the single plane of metal oxide wherein the metal of
the constituent metal oxide plane provides the large cation in the
perovskite or spinel crystalline structure.
In another embodiment of the process, the body upon which an
epitaxial film is coated has a surface defined by metal oxide
provided by either a Group IVA element oxide or an oxide
constituent of a perovskite or spinel crystal wherein the metal
element of the metal oxide provides a relatively small cation in
the crystalline form of the metal oxide and the metal and oxygen
atoms of the metal oxide are disposed at ordered sites across the
oxide surface. This process embodiment includes the steps of
growing, by MBE techniques, a constituent metal oxide plane of a
perovskite crystal or a spinel crystal epitaxially upon the single
plane of metal oxide wherein the metal element of the constituent
metal oxide plane provides a relatively large cation in the
perovskite or spinel crystalline structure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a body upon which an epitaxial
perovskite or spinel can be grown in accordance with an embodiment
of the method of the present invention.
FIG. 2 is an exploded perspective view of a structure within which
a film of perovskite is grown upon a layer of MgO and illustrating
schematically the successive layers of constituents comprising the
structure.
FIG. 3 is a schematic perspective view of ultra high vacuum
equipment with which steps of the present invention may be
performed.
FIG. 4 is a SEM micrograph image of a cross section of a
BaTiO.sub.3 film of 0.6 .mu.m thickness epitaxially grown upon
MgO(001).
FIG. 5 is a cube model representing the lattice orientation at the
interface of a structure wherein an MgO surface is covered with
BaO.
FIG. 6a is a photograph providing RHEED data for a clean MgO
surface wherein the data is obtained along a [100] zone axis.
FIG. 6b is a photograph providing RHEED data for a single layer
coverage of BaO on (001)MgO wherein the data is obtained along a
[100] zone axis.
FIG. 7a is a photograph (like that of FIG. 6a) providing RHEED data
for a clean MgO surface wherein the data is obtained along a [100]
zone axis.
FIG. 7b is a photograph providing RHEED data for one monolayer
coverage of TiO.sub.2 on MgO(001) wherein the data is obtained
along the [100] zone axis.
FIG. 8a is a plan view of a ball model of a clean MgO surface.
FIG. 8b is a plan view of a ball model of a one monolayer coverage
of TiO.sub.2 on MgO(001).
FIG. 9 is a table providing in-plane and out-of-plane structure
data and index of refraction data for SrTiO.sub.3 and BaTiO.sub.3
thin films on MgO.
FIG. 10 is a graph providing data relating to wavelength dependence
of optical loss in thin film SrTiO.sub.3 on MgO.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
Turning now to the drawings in greater detail, there is shown in
FIG. 1 a body or wafer 20 having a surface 22 defined by substrate
layer of an alkaline earth oxide, i.e. the (001) face, upon which a
perovskite or spinel of single-orientation can be grown. In the
interests of the present invention, the surface layer of the
alkaline earth oxide can be provided by the outer layer of a body
comprised entirely of the alkaline earth oxide or the outer layer
of a series of layers formed upon a base substrate comprised, for
example, of a semi-conducting material such as silicon. In either
instance, however, the crystalline structure of the alkaline earth
oxide is clean, ordered and atomically smooth to promote the
subsequent epitaxial growth thereupon of constituents of a
perovskite crystal.
The crystalline lattice structure of perovskite is
face-centered-cubic (fcc) and includes a plane of a Group IVA
element oxide, i.e. an oxide of a group consisting of TiO.sub.2,
ZrO.sub.2, and HfO.sub.2, and another plane of a different metal
oxide. For example, the crystalline lattice structure of the
perovskite BaTiO.sub.3 includes a plane of TiO.sub.2 and a plane of
BaO. Similarly, the bulk crystalline structure of the perovskite
SrTiO.sub.3 includes a plane of TiO.sub.2 and a plane of SrO. As
will be apparent, an embodiment of the process of the invention
described herein involves the initial formation of a plane of a
Group IVA element oxide upon the alkaline earth oxide surface and
the subsequent formation of additional planes of metal oxide and a
Group IVA element oxide upon the initial plane of the Group IVA
element oxide so that the subsequently-formed planes alternate with
one another.
As will be apparent herein, the crystalline lattice structure of an
oxide in the oxide class known as spinel is comparable to the
crystalline lattice structure of a perovskite, i.e. is a
face-centered-cubic, in a manner which renders the present
invention applicable to the growth of spinels, as well as
perovskites.
With reference to FIG. 2, there is illustrated an exemplary
structure, indicated 24, upon which alternating planes 26 and 28 of
the Group IVA element oxide TiO.sub.2 and metal oxide,
respectively, are formed upon the alkaline earth oxide surface 22
comprised, in this instance, of MgO. Each plane 26 or 28 is formed
upon the MgO surface 22 by molecular beam epitaxy (MBE) techniques
and with MBE equipment. Briefly, the MBE equipment with which the
process described herein can be carried out includes an ultra high
vacuum (UHV) growth/characterization facility, a fragment of which
is indicated 30 in FIG. 3. The facility 30 includes a container 32
having an inner chamber within which the body 20 is positioned so
that its surface 22 faces downwardly, and a plurality of canisters
34, 36 and 38 are provided within the base of the container 32 for
providing a vapor source of metal desired to be added to the
substrate surface during the formation of the structure 24. In this
connection, each canister 34, 36 and 38 is adapted to hold a
crucible containing a desired metal, and in this case, the
canisters hold metal constituents of the perovskite, e.g.,
BaTiO.sub.3, SrTiO.sub.3, CaTiO.sub.3 or MgTiO.sub.3, desired to be
formed upon the MgO surface 24.
An opening is provided in the top of each canister, and a shutter
is associated with the canister opening for movement between a
closed condition at which the interior of the canister is closed
and thereby isolated from the MgO surface 22 and a closed condition
at which the contents of the container 32, i.e., the metal vapor,
is exposed to the MgO surface 22. In addition, an oxygen source 40
is connected to the chamber so that by opening and closing a valve
associated with the source 40, oxygen can be delivered to or shut
off from the chamber. The opening and closing of each canister
shutter and the oxygen source valve is accurately controlled by a
computer controller (not shown).
Before the desired layers, or planes, are grown upon the MgO
surface 22, the MgO surface is rendered atomically smooth. To this
end, the MgO surface 22 can be treated with a polishing compound
which is commercially available as a cleaner under the trade
designation Syton. The body 20 is then placed within the UHV
facility 30, and the temperature of the body 20 is raised to about
1000.degree. C. At this elevated temperature, unwanted
contaminants, such as water and dirt, are driven from the surface
22 and Mg ions which may be under strain at the surface 22 are
permitted to shift to a more stable, or relieved, position. While
maintaining suitable control over the operation of the MBE facility
30, MgO is grown onto the surface 22 to restore crystalline
perfection at the MgO surface as MgO is deposited within so as to
fill voids or similar defects which may exist across the surface
22. By growing an additional thickness of about 1000 .ANG. of Mg
onto the surface 22, the desired cleanliness and smoothness of the
surface 22 is obtained.
In preparation of the growth of TiO.sub.2 onto the MgO surface 22,
the pressure in the UVH chamber is lowered to between about
2-5.times.10.sup.-7 torr. The desired layer of TiO.sub.2 is then
built upon the MgO surface 22 by conventional MBE techniques while
the chamber pressure is maintained between about
2-5.times.10.sup.-7 torr. For example, Ti metal vapor could
initially be deposited upon the MgO surface 22 and then oxygen from
the source 40 could be released over the surface so that the
desired layer of TiO.sub.2 is formed at the surface 22.
Alternatively, the surface 22 could be simultaneously exposed to Ti
vapor and oxygen, in controlled amounts, so that TiO.sub.2 forms
and then accumulates on the surface 22.
During either of the aforementioned deposition processes involving
the TiO.sub.2 layer, careful control of the MBE operation is
maintained to ensure that no more than one layer, i.e., one plane,
of TiO.sub.2 is deposited upon the surface 22. The bulk form of the
compound TiO.sub.2, as characterized by the ordered surface
structure formed in this step, has a nonequilibrium structure and
is not found in nature, and there exists a tendency for the formed
TiO.sub.2 to accumulate into clusters if the surface 22 is exposed
to a greater amount of TiO.sub.2 than is needed to comprise a
single plane of TiO.sub.2. Of course, if such clusters develop, the
TiO.sub.2 layer looses its order, and the ability to grow ordered
layers upon the TiO.sub.2 layer is destroyed. Thus, careful control
must be maintained over the deposition of Ti vapor and the release
of oxygen from the source 40 so that a single layer, and only a
single layer, of TiO.sub.2 accumulates at ordered sites upon the
MgO surface 22.
Following the development of the desired layer of TiO.sub.2 upon
the MgO surface 22, a layer of metal oxide which comprises the
other plane of the desired perovskite is formed upon the TiO.sub.2
layer. If, for example, the desired perovskite is BaTiO.sub.3, then
the vapor released in the facility chamber is Ba, and if the
desired perovskite is SrTiO.sub.3, then the vapor released into the
chamber facility is St.
Conventional MBE techniques are used to grow the desired oxide,
e.g., BaO or SrO, layer upon the formed TiO.sub.2 layer. For
example, the metal vapor, e.g., Ba or Sr, may be initially
deposited upon the TiO.sub.2 surface, and then the oxygen may be
subsequently released into the chamber so that the metal oxide
forms upon the TiO.sub.2 surface. Alternatively, the TiO.sub.2
layer could be simultaneously exposed to metal vapor and oxygen so
that the metal oxide accumulates on the TiO.sub.2 layer. In either
event, careful control should be maintained over the deposition
operation here so that no more than one plane of the desired metal
oxide is developed at this stage upon the TiO.sub.2 layer and so
that the pattern of metal oxide deposited upon the TiO.sub.3 layer
is ordered.
Upon formation of the desired plane of metal oxide, a second plane
of TiO.sub.2 is grown upon the metal oxide plane in accordance with
the aforedescribed techniques used to grow TiO.sub.2 onto the MgO
surface. Then, upon formation of the desired second plane of
TiO.sub.2, a second plane of the metal oxide, e.g., BaO or SrO, is
grown upon the second plane of TiO.sub.2.
Thereafter, layers of TiO.sub.2 and metal oxide are formed in an
alternating fashion until at least about twenty-five cell units of
the desired perovskite are grown upon the MgO surface. Dislocations
which may develop within the formed layers nucleate so as to
provide internal strain relief within the first twenty-five cell
units so that lattice strain does not appear at the surface of the
layup of planes. Thus, the surface defined by the twenty-fifth cell
unit is ordered and free of strain.
Once the strain-free surface of perovskite is formed, steps can
then be taken to grow addition layers of the perovskite upon the
build up of cell units. In this connection, subsequent growth of
the perovskite upon its strain-free bulk form is homoepitaxial,
rather than heteroepitaxial so that the characteristics of the
interface between adjacent layers of TiO.sub.2 and metal oxide are
not likely to present problems during growth. Thus, the perovskite
can be built upon itself after the initial twenty-five cell units
of perovskite are formed. To this end, the perovskite is grown
layer-by-layer upon the strain-free surface by conventional MBE
techniques to that each layer of perovskite is one cell unit high.
For example, the strain free surface may be initially be exposed to
Ti and metal, e.g., Ba or Sr, vapors and then to oxygen so that the
perovskites forms upon the strain-free surface. Alternatively, the
strain-free surface can be exposed simultaneously to the Ti and
metal vapors and oxygen so that the perovskite forms and then
settles upon the strain-free surface. In either instance, careful
control of the MBE process is maintained so that the build up of
successive layers of the perovskite is effected epitaxially.
The clarity of the resulting perovskite is realized, at least in
part, by the aforedescribed build up of alternating layers of
TiO.sub.2 and metal oxide on the MgO surface in that this build up
minimizes undersirable effects which could otherwise result from
interfacial electrostatics developed between MgO and the superposed
layers subsequently grown thereon. To appreciate the interfacial
electrostatics issue, the structure of the perovskite oxides can be
considered. The distinguishing characteristic of the perovskite
oxide class is recognized as a closest-packing of large cations and
oxygen anions arranged as stacked sheets normal to a [111]
direction. The octahedral interstices that form as a result of this
sheet-stacking sequence are in turn filled with higher valence,
smaller cations. The resulting structures are cubic with low index
stable crystal faces. The naturally occuring crystal truncations
are {001} and are then, for example with BaTiO.sub.3, either BaO
planes or TiO.sub.2 planes, as mentioned earlier. The ion sizes and
charges in these planes are distinctly different, and the
initiation of a heteroepitaxial growth sequence for such a
structure on another insulating oxide must take this into
account.
With reference to the micrograph image of FIG. 4, there is shown a
fracture cross section of a representative BaTiO.sub.3 film on
(100)MgO. The FIG. 4 material was grown by using source-shuttering
MBE techniques in ultra high vacuum. The film is adherent, single
phase and optically clear. The epitaxy is cube-on-cube and uniquely
results from the aforedescribed layering sequence that begins at
the TiO.sub.2 -plane of the perovskite structure. The layering
sequence is a requirement for single-orientation, epitaxial growth
of a perovskite on MgO.
For a heteroepitaxial transition between insulating oxides, the
interface electrostatics (ion--ion near neighbor interactions) of
the first layers critically determine whether a commensurate
structure can develop. For example, in going from MgO to
BaTiO.sub.3 on the (001) face of MgO, if the transition is
initiated at a barium oxide plane, the structure at the interface
cannot develop commensurately with the MgO surface. The basic
incompatibility results from the large ion-size difference between
barium and magnesium. In particular, it is impossible to avoid
near-neighbor ion configurations where cation--cation or
anion--anion repulsive interactions occur in large numbers. This
naturally leads to interfacial energy and an inherent instability.
In each study made up until now which has been directed to
interfacial equilibrium and surface segregation phenomena for the
alkaline earth oxides, the clear result emerged that no single
layer of BaO on MgO existed that was energically stable. We have
found that the energetic stability is of paramount importance to
the growth of single-orientation perovskites on MgO.
For purposes of comparison, barium metal and oxygen was deposited
onto a MgO surface at a substrate temperature of 500.degree. C. to
form BaO at a 1/2 monolayer coverage based on the MgO surface. This
monolayer coverage is equivalent to one monolayer of BaO in
BaTiO.sub.3. The high interfacial energies that would result from
commensurate BaO epitaxy on MgO should drive some mechanism for
lowering the interfacial energy. In this regard, there is shown in
FIG. 5 a cube model of the interface and associated reflection
high-energy electron diffraction (RHEED) patterns from clean and
1/2 monolayer BaO-covered (100)MgO surfaces. The implication of
surface segregation theories is that island-like nucleation of
incommensurate BaO-type structures should develop, and it is
believed that this does occur. The cube model shown in FIG. 5 shows
an idealization of parallel and 45.degree.-rotated morphologies of
an (100) interface between MgO and BaO, and FIGS. 6a and 6b show
diffraction patterns as experimental confirmation of their
existence. The RHEED pattern shown in FIG. 6a results from an MgO
surface prepared in the MBE system by growing 100 nm of MgO
homoepitaxially on (001)MgO. The 0,0 and allowed 0,2 surface rods
are seen. In FIG. 6b, surface diffraction at the same zone axis is
illustrated but is modified by a single-layer-coverage BaO
deposition. It can be seen in FIG. 6b that incommensurate
crystallite orientations have formed and give rise to diffraction
at what would be the 0,2 rod position for cube-on-cube BaO and at
the 1,1 rod of 45.degree.-rotated BaO as well. Moreover, in
addition to the rod spacing indicating the microstructural
characteristics of the interface, the diffraction intensity is
modulated along the reciprocal lattice BaO rods in a Bragg-like
manner, i.e., 3-dimensional diffraction occurs that is indicative
of "islanding" or surface roughening. These multi-orientation,
3-dimensional island structures defeat any attempt at growing
optical-quality, thick perovskite films.
With reference again to the construction of the structure of the
present invention, there are provided in FIGS. 7a and 7b
photographs of RHEED data which illustrate the dramatically
different result that can be obtained by moving up one plane from
the MgO layer (whose ball model is depicted in FIG. 8a) in the
BaTiO.sub.3 unit cell to the TiO.sub.2 plane (whose ball model is
depicted in FIG. 8b) and initiating the growth sequence at that
point. A commensurate, atomically flat layer of TiO.sub.2 can form
in which every other cation row is vacant over the underlying
Mg.sup.2+ sites. This TiO.sub.2 surface satisfies the electrostatic
requirements for anion-cation near-neighbor pairs at the interface
and is a low-energy, stable truncation of the MgO surface. The
missing row of cations in this layer provides the energetically
favorable sites for subsequent barium ion attachment to the crystal
surface. As the perovskite growth is continued with alternating
barium and titanium deposition cycles, BaTiO.sub.3 grows
layer-by-layer and strain relief can occur by nucleation of simple
edge dislocations maintaining the single orientation cube-on-cube
epitaxy. The BaTiO.sub.3 lattice parameter relaxes to its
strain-free, bulk value within ten unit cells from the original
interface. The transition from heteropitaxy to homoepitaxy of the
perovskite is completed with the desired single-orientation
material and its advantageous long-range structural coherence. With
the transition from heteroepitaxy to homoepitaxy accomplished in
the manner described above, growth rates on the order of 1 .mu.m/hr
can be attained at temperatures as low as 500.degree. C. by
codeposition of barium and titanium or strontium and titanium with
oxygen arrival rates equivalent to pressures of 10.sup.-7 torr.
Structural and optical characteristics of the resulting thin films
are provided in table form in FIG. 9.
The MBE process described above for the stabilization of the
interface between a perovskite oxide and the alkaline earth oxide
MgO provides an opportunity heretofore unavailable to exploit the
electro-optic properties of thin-film epitaxial ferroelectrics in
waveguide applications. In support of this contention, there is
provided in FIG. 10 a plot of the waveguide dependence for optical
loss in thin film SrTiO.sub.3 on an MgO surface. Such a film is of
high optical clarity and can be directly compared with the
performance of LiNbO.sub.3, the most commonly applied material in
EO devices. It is believed that this is the first demonstration of
such optical clarity of SrTiO.sub.3 and BaTiO.sub.3 grown in thin
film form. The crystal quality that is obtained by the methods
described above does not result from incremental improvements upon
known information, but rather, is attained by directly addressing
the fundamental requirements of interfacial energy minimization
between perovskite and alkaline earth oxides.
It will be understood that numerous modifications and substitutions
can be had to the aforedescribed embodiments without departing from
the spirit of the invention. For example, although the
aforedescribed process describes a build up of a relatively thick
film of perovskite upon a MgO surface, a usable product which
could, for example, permit the intrinsic properties of MgO to be
studied may include only a single layer of TiO.sub.2 overlying a
MgO surface. Thus, in accordance with the broader aspects of the
present invention, an embodiment of the process could terminate
upon the formation of a single plane of TiO.sub.2 (or another Group
IVA element oxide) upon a MgO (or other alkaline earth oxide)
surface.
Still further, although the aforementioned embodiments have been
described in connection with perovskites which include a plane of
titanium oxide (TiO.sub.2), the principles of the present invention
are applicable to other perovskites and oxides in the class of
oxides known as spinels. The distinguishing structural
characteristic of the perovskite or spinel oxide class with which
this invention is concerned is recognized as a closest-packing of
large cations and oxygens arranged as stacked sheets, and between
these sheets are positioned higher valence, smaller cations. For
example, in each of the perovskites BaZrO.sub.3, SrZrO.sub.3 and
PbZrO.sub.3, the metal zirconium provides the small cations in the
crystalline structure (and bonds with oxygen in one plane of the
structure to form ZrO.sub.3) while the metal element Ba, Sr or Pb
provides the larger cations. Similarly, in the perovskite
SrHfO.sub.3, the metal hafnium plays the role of the small cations
while the metal strontium plays the role of the large cations.
Along these lines, the metal oxide plane of a perovskite crystal
containing the small cation can be comprised of a mixture of
different, albeit suitable, e.g. Group IVA, elements. For example,
the perovskite BaTi.sub.x Zr.sub.1-x O.sub.3 can be built
epitaxially upon a substrate of MgO (or another alkaline earth
oxide) in accordance with the principles of the present invention
wherein titanium and zirconium are used in the construction of the
crystalline planes of the perovskite structure which include the
small cations. The perovskites are generically in the stochiometry
of ABO.sub.3 wherein A is an element like Mg, Ba, Sr, Ca and Pb,
all of which have valence states of +2, and B is an element like
Ti, Hf or Zr having valence states of +4.
Similarly, the crystal structure of an oxide known as a spinel is
known to include a face whose lattice structure, when viewed
frontally, simulates that of the crystalline form of a Group IVA
oxide (see, e.g. the ball model of TiO.sub.2 depicted in FIG. 8b).
In other words, these spinel oxides are provided with a constituent
oxide plane wherein the metal element of the oxide in the plane
provides a relatively small cation with respect to the size of the
oxygen in the crystalline form of the oxide and the metal and the
oxygen atoms of the metal oxide are disposed at ordered sites
across the oxide surface. The spinel oxides are provided with a
second constituent oxide plane wherein the metal element of the
oxide in this second plane provides a relatively large cation in
the crystalline form of the oxide. The spinels are generically in
the stochiometry of A.sub.2 BO.sub.4 where A is an element, i.e. a
large cation element, that is not magnetic, such as Mg, Ba, Sr, Ca
and Pb. These elements all have filled outer shell electron
configurations so that there are no unpaired electrons that give
rise to permanent magnetic moments. B is an element, i.e. a small
cation element, that can be magnetic, such as Ti, V, Cr, Mn, Fe,
Co, Ni and Cu. These latter-mentioned elements come from the
transition or rare earth element series and have unfilled inner
electron shells containing unpaired electrons which are then
responsible for their permanent magnetic moments. The magnetic
moments associated with these "B" elements undergo order/disorder
phenomena associated with ferromagnetic phase transformations and
then exhibit magneto-optic properties. These properties can be
taken advantage of in a number of devices, and in particular, are
believed to be well-suited for use in Faraday Rotators for optical
isolators and in magnetic memory applications.
It is believed that due to the aforediscussed similarity in the
crystalline forms of the Group IVA element oxides, the perovskites
and the spinels, a perovskite, a spinel or a constituent oxide
plane of a perovskite or a spinel can be grown upon a surface
provided by either of the Group IVA element oxides or an oxide
constituent of a perovskite or spinel in accordance with the
principles of the present invention. To this end, MBE techniques
are used to grow an initial constituent plane of a perovskite or
spinel crystal epitaxially upon the metal oxide wherein the metal
element of the constituent oxide plane provides a large cation in
the perovskite or spinel structure. The build up of epitaxial
layers can then be continued (e.g. toward the formation of
perovskite in bulk or spinel in bulk) by growing, with MBE
techniques, a second epitaxial layer upon the initial layer wherein
the second epitaxial layer is comprised of a constituent metal
oxide plane of the perovskite or spinel wherein the constituent
metal oxide plane of the second epitaxial layer includes the metal
element which provides the small cation in the perovskite or spinel
crystalline structure.
Accordingly, the aforedescribed embodiments are intended for the
purpose of illustration and not as limitation.
* * * * *